Studies on the functional relationships between the urinary and lymphatic systems in health and disease are limited by the lack of precise knowledge on the intrarenal pattern and distribution of the lymphatic circulation. In particular, conflicting theories and evidence exist on the presence or absence of renal medullary lymph vessels, and on the extent to which lymphatic capillaries are related to individual parts of the nephron. A new and comprehensive morphological and functional approach to the origin and formation of renal lymph, based on preliminary experiments, is proposed. The objective of the morphologic study is to depict in detail the distribution which these vessels have to vascular, tubular and connective tissue components of the dog and rat kidney. The methods include light microscopy of large-faced methacrylate embedded material and electron microscopy. Experimental procedures will be used to dilate and demarcate the intrarenal lymphatics for ease of identification and diagramatic reconstructions will be made from serial sections. The purpose of the physiologic studies is to analyse the composition of simultaneously collected samples of hilar and capsular lymph and peripheral and renal venous plasma with respect to those substances whose concentrations might be expected to differ according to the particular site of lymph formation. These substances include electrolytes, urea, glucose, inulin and PAH. Information on the sites of formation of renal lymph derived from this analysis will be correlated with the morphologic finding in order to provide a combined structural and functional concept of the origin of renal lymph.